Printed antenna structures, also referred to as printed circuit board antenna structures, are widely used to provide compact antennas that can be integrated with other microelectronic devices on a substrate. For example, printed antenna structures may be used with cellular radiotelephones, portable computers and other compact electronic devices.
Printed antenna structures often include a center feed dipole antenna that can provide omnidirectional radiation. The center feed dipole antenna is a balanced device. Since the input to the antenna is typically provided by an unbalanced input, a balanced-to-unbalanced converter, also referred to as a "balun", is also generally provided. See, for example, IBM Technical Disclosure Bulletin, Vol. 40, No. 6, June 1997, pp. 127-130 entitled "Printed Dipole With Printed Balun".
It is also often desirable to provide a printed antenna structure that can operate in multiple bands. For example, a cellular telephone may operate in a conventional analog (800 MHz) band and also in a PCS band at around 1900 MHz. It is desirable to provide a single antenna structure that can operate in both bands. For example, U.S. Pat. No. 5,532,708 to Krenz et al. entitled "Single Compact Dual Mode Antenna" discloses a printed circuit board antenna that includes an electronic switch, so that a single compact radiating structure consisting of a split dipole antenna with associated balun structure may be selectively driven in either of two modes.
As cellular telephones, PCS devices and computers become more compact, there continues to be a need for more compact printed antenna structures including baluns. There is also a continued need for compact printed antenna structures including baluns that can operate in at least two bands.